Structured materials are often synthesized through hydrothermal reaction processes, i.e., heating the liquid-phase precursor material under a certain temperature profile. To keep the solvent in the liquid phase, the heating is typically conducted in a closed or pressurized reactor vessel, e.g. an autoclave reactor. Examples of structured materials include zeolites, meso-porous silica and alumina, and metal organic frameworks. Zeolites are crystalline materials mostly comprising aluminum-silicon metal oxide compounds and have found industrial applications. A good amount of fundamental knowledge on synthesis chemistry and properties of zeolite materials has been obtained (see “Handbook of Zeolite Science and Technology” eds. by Scottm Auerbach, Kathleena Carrado, and Prabirk Dutta, 2003 Marcel Dekker, Inc.). By contrast, innovations on the reactor apparatus and processes to produce zeolite materials at large scale are very limited. Almost all hydrothermal syntheses are conducted in cylindrical autoclave reactors. An internal mechanical stirrer may be used to enhance mass and heat transfer. However, uniform heating of such reactors at large scale is difficult. On the other hand, temperature has large impact on quality and productivity of the zeolite synthesis. At the same growth time, the crystallinity increases with growth temperature. At the same crystallinity level, crystal linear growth rate rapidly increases with temperature. If the growth rate is described by a simple Arrhenius equation, the resulting apparent activation energy is 44 to 80 kJ/mol for A-type zeolite, 50 to 65 kJ/mol for Faujasite-type zeolite (X and Y), 58 to 63 kJ/mol for Mordenite, 50 to 90 kJ/mol for Silicalite, and 80 to 90 kJ/mol for ZSM-5.
Zeolite synthesis by microwave heating has been explored to enhance productivity (Wm. Curtis Conner, and Geoffrey Tompsett, Kyo-Ho Lee, and K. Sigfrid Yngvesson. “Microwave Synthesis of Zeolites: 1. Reactor Engineering” J. Phys. Chem. B, 108 (37) (2004)13913-13920). A recent report shows that zeolite crystals can be grown within a very short time by rapid mixing and rapid heating to exceptionally higher temperatures (300-370° C.) than what is typically used in the autoclave reactor (100-180° C.) (Zhendong Liu, Kotatsu Okabe, Chokkalingam Anand, Yasuo Yonezawa, Jie Zhu, Hiroki Yamada, Akira Endo, Yutaka Yanaba, Takeshi Yoshikawa, Koji Ohara, Tatsuya Okubo, and Toni Wakihara, “Continuous flow synthesis of ZSM-5 zeolite on the order of seconds” Proc Natl Acad Sci of USA; 113(50): 14267-14271; 2016 Dec. 13).
The microwave heating and micro-reactor continuous synthesis studies demonstrate that the zeolite synthesis rate can be dramatically increased by rapid heating of the growth solution and/or raising the growth temperature. Uniform zeolite growth is another important consideration to obtain zeolite crystals of uniform sizes and crystallinity. The uniformity becomes particularly desirable for growth of zeolites on large structured bodies, such as zeolite membranes and films on a large support structure of planar or tubular form. Different from zeolite synthesis in powder form, non-uniform growth of a zeolite membrane can result in complete failure of the whole structured body. The zeolite membrane could lose its molecular separation functions if incomplete growth occurs on some areas or spots. Mechanical stirring becomes ineffective for growth of a large structured body in a conventional cylindrical reactor.